House Oversight and Government Reform Committee Chairman Darrell Issa (R., Calif.) arrives for a hearing on Capitol Hill in Washington on June 23, 2014. (Nicholas Kamm/AFP/Getty Images)

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House Oversight Committee Chairman Darrell Issa is pushing ahead with his subpoena of a top presidential adviser, saying Tuesday that he still has unanswered questions about the White House political office.

White House staff briefed Mr. Issa’s staff Tuesday, answering questions about the White House Office of Political Strategy and Outreach. Mr. Issa, a California Republican, did not attend the briefing but told the White House afterward that presidential adviser David Simas must testify at a Wednesday hearing examining the scope of White House political activities.

“The briefing provided to committee staff today, while helpful and appreciated, did not answer all questions and did not put the White House on the record about changes made to its political office,” Mr. Issa wrote to White House Counsel W. Neil Eggleston. “The committee has outstanding questions for Mr. Simas, who did not take part in the briefing, and it is necessary for him to appear at tomorrow’s hearing.”

The White House has pushed back against the subpoena, asking Mr. Issa to lift it immediately. Mr. Eggleston argued in a letter Monday that summoning Mr. Simas to testify raised significant concerns about separation of powers, adding that administration officials from both parties have held that view in the past.

Democratic and Republican presidents have maintained political offices in the White House. Mr. Obama closed the White House’s Office of Political Affairs in 2011 and re-launched the current iteration of the political office in January.

Mr. Issa has raised questions about whether the White House’s political office has abused taxpayer funds for political gain in violation of the Hatch Act.

“In prior administrations, White House political officials have encountered difficulties squaring their activities with the prohibitions on political or campaign-related activities of federal officials outlined in the Hatch Act,” Mr. Issa wrote in a letter Tuesday. “It strains credulity that this administration has, in contrast to its predecessors, uniquely resolved all concerns about political activity and should not be subject to the same level of congressional oversight requests for testimony and documents as previous administrations.”

Rep. Elijah E. Cummings of Maryland, the ranking Democrat on the House Oversight Committee, said Tuesday’s briefing on the White House political office was detailed and thorough and that there was no evidence Mr. Simas or his staff did anything wrong.

“There seems to be no reason to continue this ridiculous confrontation other than to manufacture false controversy as Chairman Issa’s tenure comes to an end,” Mr. Cummings said in a statement.

The House Oversight Committee hearing is scheduled for 10 a.m. Wednesday.

Mr. Issa’s refusal to drop the subpoena set off a flurry of correspondence between the White House and Mr. Issa’s office Tuesday evening.

Mr. Eggleston sent Mr. Issa a letter asserting that Congress could not compel the president’s top advisers to testify.

“Mr. Simas is immune from congressional compulsion to testify on matters relating to his official duties and will not appear at the July 16, 2014 hearing,” the White House counsel wrote.

Mr. Eggleston included an opinion from the Department of Justice’s Office of Legal Counsel arguing that Mr. Simas is not required to testify. Compelling the testimony of the president’s closest advisers would damage the separation of powers and would threaten executive branch confidentiality, which is necessary to ensure that the president can obtain sound and candid advice, the opinion says.

Mr. Issa soon responded, saying the White House’s claim that senior advisers are immune from the congressional process had already been rejected by a federal court.

“Flouting a federal judge’s opinion about our system of checks and balances is yet another attack on our nation’s constitution by this president,” Mr. Issa said.

The Republican lawmaker said he still planned to convene Wednesday’s hearing “to ensure that Mr. Simas has an opportunity to comply with his legal obligation to appear.”

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